Television signal-reproducing system



Aug. 19, 1941. J. c. wlLsoN TELEVISION SIGNAL-REPRODUJING SYSTEM FiledMarch 2. 1939 vvvvvv Patented ug. 19, 1941 OFFICE TELEVISIONSIGNAL-REPRODUCING SYSTEM John O. Wilson, Bayside, N. Y., assigner toHazeltine Corporation, a corporation of Delaware Application Merch z,1939, serial No. 259,380

4 claims. (ci. 17a- 7.5)

This invention relates to television signalreproducing systems andespecially to such systems by means of which images may be faithfullyreproduced irrespective of certain operating conditions tending toimpair the fidelity of reproduction.

In accordance with present television practice, atransmitted signalcomprises a carrier wave, modulated during successive intervals or traceperiods by Vvideo-frequency and unidirectional components representativeof variations in illumination of successive incremental areas of animage being transmitted andof its average background. illumination,respectively. Between the trace periods, that is, during retraceintervals, the carrier wave has a predetermined amplitude level referredto as a blanking level and corresponding tol al given shade, usuallyblack, and is modulated during a part of this retrace period bysynchronizingsignal components which correspond toinitiations ofsuccessive lines and fields in the scanning of the image.

At the receiver, a beamis so deflected as to scan and illuminate atarget or screen in a series of fields of parallel lines. Thesynchronizingsignal components of the received signal are separated fromthe other modulation-signal components and utilized to control thescanning apparatus of the receiver so as to synchronize its operationwith that of similar apparatus utilized at the transmitter in developingthe signal. The intensity of the beam. is controlled. by thelight-modulation. components, thereby to reconstruct the image. l

Since the eye of an observer is responsive primarily to contrasts orratios between illumination values rather than absolute value of anygiven shade of illumination of an object, such as black, it is importantthat the extreme values of illumination in the reproduced imagecorresponding to signal components representing white and black in thetransmitted image have a constant ratio under al1 operating conditions.That'is, the contrast ratio of the reproduced image should be maintainedconstant for a given amplitude of a signal to be reproduced irrespectiveof variations in the signal level corresponding to such given shadevalue.

To this end it is attempted in conventional television receivingsystems-so to control the system that the modulation signal is stabilized withrespect to a particular shade value or level, such as the blankinglevel. More particularily, it is attempted so to control the system thata given signal level or amplitude corresponding to black, or anypredetermined shade value, is effectively held fixed with respect to thesignalinput brightness characteristic of the reproducing device of thesystem, in order that signal components representing-any other givenshade will at all times appear as the same shade in the reproducedimage. However, due to various operating conditions, for example,variations in the average signal amplitude which are not compensatedfor, or to failure of the stabilizing circuits completely to fulfilltheir function, the signal level corresponding to black as applied tothe reproducing device frequently is not maintained at a fixed value.Under these conditions, therefore, and in view of the fact thatconventional television reproducing systems have either linear orlogarithmic signal-input brightness characteristics, the contrast ratiotends to'vary, thereby impairing the fidelity of reproduction. Ingeneral, the system should be controlled so that the light gradations orshades from black to white, represented by the various light-modulationcomponents, are all faithfullyrepresented in the reconstructed image, agiven light gradation being represented by a constant contrast ratiounder all operating conditions.

It is an object of the present invention, therefore, to provide animproved television signal' reproducing system whereby the contrastratio for a given signal-input amplitude is maintained constant underall operating conditions.

In accordance with a feature of the present invention, there is provideda television signalreproducing system which comprises signalvaries inaccordance with operating conditionsA in ther system, which variationstend to effect undesired variations of the contrast ratio of areproduced signal of a given amplitude. There are further provided meansincluded in .said

reproducing means forimparting to the repro- Y ducing means anexponential signal-input brightness characteristic over the operatingrange of said sytem eective to provide a substantially constant contrastratio for a reproduced signal of a given amplitude irrespective of theblack-level variations.

Also in accordance with a feature of vthe invention, there is provided,in a television signalreproducing system, a cathode-ray televisionsignal-reproducing tube including an electron gun for developing,accelerating and focusing an elec tron beam, a fluorescent target in thepath of pitch providing an exponential signal-inputbrightnesscharacteristic for the tube.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing, and itsscope will be pointed out in the appended claims.

In the accompanying drawing, Fig. l is a circuit diagram, partiallyschematic, oi a television receiver including a signal-reproducingsystem embodying the present invention, while Figs. 2 and 3 are curvesillustrating certain characteristics of the receiver of Fig. l.

Referring now more particularly to Fig. l of the drawing, the systemillustrated comprises a receiver of the superheterodyne type includingan antenna system Ill, II connected to a radiofrequency amplifier I2, towhich there are connected in cascade, in the order named. anoscillator-modulator I3, an intermediate-frequency amplifier Il, adetector I5, 'a video-frequency amplifier I6, and a signal-reproducingsystem I1, which, in this embodiment, comprises a cathode-raysignal-reproducing tube I8. The cathode-ray tube I8 comprises the usualenvelope provided with an elongated neck containing an electron gun,constructed in accordance with the present invention as hereinafter morefully described, including a cathode IB having an external heater 20, aninternal signal-input or con trol grid 2|, a screen 22, a first anode23, and a second anode 24. 'I'he tube also includes the usualfluorescent screen 25 on theenlarged end of the tube opposite theelectron gun. Linefrequency and field-frequency scanning coils 28 and 21are disposed about the neck ofthe tube for deilecting the beam in twodirections normal to each other. The output circuit of thevideofrequency amplier I6 is connected to the control grid 2I by way ofa leak resistor 29. Operating potentials for the several electrodes ofthe device I1 are supplied from a suitable source, as represented by thevoltage-divider resistor 30.

A line-frequency generator SI and a'iieldfrequency generator 32 arecoupled to the output circuit of the detector I for synchronization andare connected to the scanning coils 28 and 2l, respectively, of thecathode-ray tube in the conventional manner. Suitable means are includedin the generators 3i and 32 for separating the synchronizing'pulses fromthe other modulation components and from each other. The stages or unitsJust described, excepting the cathode-ray tube, may all be ofconventional well-known construction so that detailed illustrations anddescriptions thereof are unnecessary herein.

Referring briey to the general operation of the system described aboveas a whole, television signals intercepted by the antenna circuit I0, Ilare selected and amplied in the radio-frequency amplier I2 and suppliedto the oscillator-modulator I3 wherein theyare converted tointermediate-frequency signals which, in turn, are selectively amplifiedin the intermediatefrequency amplier I4 and delivered to the detectorI5. The modulation-signal components of the carrier wave are derived bythe detector I5 and are supplied to the video-frequency amplier I6wherein they are amplied and from which they are supplied to the controlgrid ill ci the cathode-ray tube Il.

With proper operating potentials supplied from the source d@ to theelectrodes of the tube i8, an electron beam is emitted from the electrongun, its intensity being controlled by the grid 2i in accordance withthe video-frequency voltages impressed thereon and in a mannerhereinafter explained in detail, which is in accordance with the presentinvention.

Saw-tooth current waves are generated in the line-frequency andfield-frequency generators 3l and 32, which are controlled by thedetected synchronizing pulses, and these waves are applied to thescanning elements 26, 2 of the cathode-ray tube I8 to produce electricscanning elds, thereby to deflect the ray in twoidirections normal toeach other so as to trace a rectilinear scanning pattern on the screenof the ltube and thereby to reconstruct the transmitted picture.

Referring now more particularly to the portion of the system of Fig. lin which the present invention is embodied, it will be appreciated thatheretofore it was attempted to provide signalreproducing systems ortubes with input-signal brightness characteristics which weresubstantially linear or which closely approached linearity. Thischaracteristic of most conventional cathode-ray tubes actually has beena three-halves power curve.` Furthermore, as has been stated, in theoperation of television signalreproducing systems, it frequently occursthat the level of the signal representing' black, as applied to thereproducing means, is not maintained at a xed value even though someother amplitude level related to black, such as the level representingthe peaks of the synchronizing components, does remain substantiallyfixed. In such arrangements, therefore, black-level variations haveeffected undesired variations of the contrast ratio of the reproducedsignal. These conditions are illustrated by the curves of Fig. 2 whereincurve A represents the signal-input brightness characteristic of areproducing system of the type employed in the prior art. The wave formsof an applied input signal are shown at V1 and V2 which represent thesame signal under diierent operating conditions, V2 being identical withV1 except that it is stabilized at the level Stn, while the signal V1 isstabilized at the level Sti. in the wave forms V1 and Va, the portions Srepresent the synchronizing pulses which occur during the retraceperiods of the 'wave while the portions V represent the video-frequencycomponents, the levels indicated at b1 and b2 correspending to black andthe levels indicated at w1 and ws corresponding to full white in theimage being transmitted. Brightness values in the reproduced imagescorresponding to the levels b1, bz, wi, and wz are indicated at bin,h2o, w10, and w20, respectively. It will be seen that for the signal V1stabilized at Str, the contrast ratio 101/1210 is of the order of 1p tol. However, for the signal V2 stabilized at St'z, the contrast ratiowao/b2o is substantially reduced, being of the order of 5 to 1. 'I'hereis, therefore, a very appreciable distortion of the image reproducedfrom wave V: as it appears Ato an observer whose eyes are chieiiyresponsive to the ratio of the limiting brightness values, asdistinguished from absolute values, as explained above.

To compensate for these undesired variations in the contrast ratio,therefore, the signal-reproducing device I8 is designed in accordancewith the present invention. To this end, the cathode able thermionicemssive materiaL ness levels.

I9 of the electron gun of the tube I8, in the preferred embodimentillustrated in Fig. 1, comprises a sleeve of very small' diametercentrally located in the end of the neck \of the tube envelope. Thecathode is preferably constructed/of nickel and its inner surface iscoated with suit- The lamentary cathode heater 2u preferably comprises ahelix of tungsten wire disposed externally around the cathode. For thepurpose of controlling the intensity o'f the electron beam developed by.the gun, the control electrode 2l is disposed within the cathode sleevea small distance from its inner surface. This electrode comprises ahelical lgrid havinga nonuniform pitch, preferably a pitch whichprogressively increases thro'ughout its length, or at least a portionthereof, as shown in the drawing.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is: j

1. A television signal-reproducing system comprising, signal-reproducingmeans, means for developing and applying to said reproducing means Theelectron gun structure constructed as described above imparts. to thetube I8 an exponential signal-input brightness characteristic such asillustrated by curve B of Figsl 3 )and represented by the equation: Y

v =ee where =the 'illumination of the tube Atarget and e=thesignal-input voltage.

That i's, the electron gun includes means for imparting to thesignalfreproducing tubean exponential signal-input brightnesscharacteristic overthe operating range of the system eiiective toprovide a substantially constant contrast ratio for a reproduced signalof given amplitude irrespective of black-level variations thereof. Thislis4 clearly illustrated in Fig, 3 ,where'-the. wave forms Viiand- V2.arev thesame as those shown in Fig. 2 and similar'designations areemployed for the varioussignal components and bright- It is seen that,due vto the exponential signal-input brightness characteristic of curveB, the contrast ratiowzo/bzofor the signal V2 is substantially thesameas the contrast ratio 'uno/lno for the signal V1,irrespective.of thefact that the black level b2 of the signal Vzis considerably greaterthan. the black level b1 of the signal Vi.

' It will be appreciated that, while a particular means has been shownasthe preferred embodiment of the invention for imparting the desiredsignal-input brightness characteristic to the signal-reproducing systemfor 'compensating for the black-level signal variations, various othersuitable means may be provided in accordance with the present inventionfor accomplishing this purpose. For example, instead of utilizing thenovel type of cathode-ray signal-reproducing tube described above, thedesired characteristic may be imparted to the signal-reproducing meansby utilizing a conventional type of cathode-ray tube having` anapproximately linear characteristic and including in the reproducingmeans an ampliner tube having the required exponential characteristicand having a direct-current coupling to the cathode-ray tube.

a signal having an amplitude level related to black which issubstantially fixed and having an amplitude level corresponding to blackwhich varies in accordance with operating conditions in said system,such variations tending to Ieffect undesired variations of the contrastratio of a reproduced signal of given amplitude, and means included insaid reproducing means for imparting to said reproducing system anexponential signal-input brightness characteristic over the operatingrange of said system effective to provide a substantially constantcontrast ratio for said reproduced signal irrespective of said blackflevel variations. y

2. A television signal-reproducing system comprising an input circuitadapted to have a signal applied thereto having an amplitude' level re'lated to black which is substantially fixed and having an amplitudelevel corresponding to black which varies in accordance with operatingconditions in said system and a cathode-ray signalreproducing tubecoupledto saidinput circuit,

said black-level variations tending to effect undesired variations ofthe contrast ratio of a signal as reproduced by said tube, said tubecornprising an electron gun having a control electrode to impart to saidtube an exponential signal-input brightnesscharacteristic over `theoperating range of said system effective to provide a substantiallyconstant contrast ratio for said reproduced signal irrespective of saidblack-level variations.

3. In a television signal-reproducing system, a

cathode-ray television signal-reproducing tube including an electron gunfor developing, accelerating, and focusing .an4 electron beam, a flu()-rescent target in-the path of said beam, andal i helical signal-inputelectrode included in said gun and having nonuniform pitch providing anlexponential signal-input brightness characteristic for said tube.

4. In a television signal-reproducing system, a cathode-ray televisionsignal-reproducing tube including an electron gun for developing,accelerating. and focusing an electron beam, av fluorescent target inthe path of said' beam, and a helical signal-input electrode included insaid gun having a progressively increasing pitch throughout at least aportion of its extent providing an exponential signal-input brightnesscharacteristic for said tube.

JOHN C. WILSON.

